Trap Nights
Running an actinic Skinner fifty metres from an MV Robinson for one September night
Beginners are told a small actinic trap catches ‘fewer moths, but the same ones’. On a warm, cloudy night in a Herefordshire orchard I ran both at once to see how true that is.
On the night of 29 September I ran two traps in the old cider orchard behind the house at Much Marcle. One was a Robinson with a 125-watt mercury-vapour bulb at the hedge end; the other was a wooden Skinner with a 15-watt actinic tube, fifty metres away under the big perry pear. Both went on at 18:55, four minutes after sunset, and both were emptied at 06:40 the next morning. I wanted to test something people tell newcomers almost as a reassurance: that an actinic trap catches ‘fewer, but the same things’. One night proves nothing on its own, but I wanted real numbers to set against the saying.
Conditions were about as good as late September offers. Cloud sat at seven oktas from dusk until a little after 02:00, and the minimum on the max–min thermometer in the hedge was 11 °C. The wind was a light south-westerly, force 2 at most. It was three nights after the full moon of 26 September, so a waning gibbous moon was up for most of the night. On a clear night it would have cut the catch noticeably; under that much cloud I doubt it mattered. Warm cloud counts for more than moonlight in autumn, and this was a warm, cloudy night.
The Robinson held 312 moths of 34 species. The Skinner held 97 moths of 21 species. Nineteen species turned up in both traps, fifteen only in the MV and two only in the actinic. Most of the difference in numbers came from a handful of common noctuids. There were 68 Large Yellow Underwings (Noctua pronuba) in the Robinson against 11 in the Skinner, 54 Lunar Underwings (Omphaloscelis lunosa) against 23, and 41 Square-spot Rustics (Xestia xanthographa) against 19. Leave those three out and the gap narrows a great deal: 149 moths against 44, rather than 312 against 97.
The two actinic-only species were the most interesting result of the night, though not because of the lamp. One was a single Brick (Agrochola circellaris). The other was a pair of Acleris sparsana, a sycamore-feeding tortrix that the MV has not recorded here in four autumns of trapping. The Skinner stood six metres from the only sycamore on the property, and the Robinson stands nearly sixty. These two moths say much more about where a trap is placed than about what kind of light it uses, and that is a variable most comparisons, including this one, fail to control.
The MV took the night’s best moths. There were three Merveille du Jour (Griposia aprilina), always the first sign that the oaks along the lane are doing their job. Two Dark Sword-grass (Agrotis ipsilon) and a single Vestal (Rhodometra sacraria) were almost certainly carried up on the south-westerly. All three migrants arrived after midnight, by which time the actinic had gone nearly quiet. The arrival times on the chart above come from hourly checks of the Robinson with a red head torch until half past one. After that I went to bed, so later times are only the earliest individual I could place from the egg trays in the morning. Treat those as estimates.
My practical conclusion is a modest one. If you have neighbours, or a small garden, or a single leisure battery, a 15-watt actinic will on a good night give you something like three-fifths of the species list an MV would. Here it was 21 of 34, or 62 per cent. It will give you nothing like the same counts. If you are recording abundance, or hoping for migrants on a southerly airflow, the MV earns its glare. I logged the two traps as separate entries in iRecord at SO6532, with lamp type in the comments, so the county recorder can separate them. In October I will repeat the night three times with the two traps swapped between hedge and pear, which is the only way to separate the effect of the lamp from the effect of the tree.